As is known in the art, power conversion systems that interface between direct current (DC) and single-phase alternating current (AC) require an energy storage capability (or an energy buffer) which provides buffering between a constant power desired by a DC source or a load and a continuously varying power desired for a single-phase AC system.
As is also known, the flow to and from such an energy buffer is at twice the line frequency (e.g., 120 Hz in the United States). The buffering energy requirement can be calculated as Ebuf=P/ωline. Because the energy storage requirement of the buffer is proportional to the system average power (P) and the (relatively long) line period (T=2π/ω), the size of the required energy buffer cannot be reduced simply through increases in switching frequency of an interface power converter. Thus, energy buffering requirements represent a significant limitation on miniaturization of grid interface systems.
One important consideration associated with twice-line-frequency energy buffering relates to lifetime and reliability. Conventional power conversion systems typically utilize electrolytic capacitors to provide high-density energy storage for buffering. It is, however, widely appreciated that despite providing the best available energy density and providing small DC bus voltage variation, electrolytic capacitors also represent a significant source of system lifetime and reliability problems. Also, electrolytic capacitors can only be operated over a narrow charge/discharge range at 120 Hz for thermal and efficiency reasons (i.e., associated with RMS current limits and efficiency requirements). These considerations directly limit the energy buffering capability of electrolytic capacitors at 120 Hz. Thus, while typical peak energy storage densities of up to 0.9 J/cm3 can be achieved with electrolytic capacitors, the allowable energy swing at 120 Hz yields practical energy densities that are about an order of magnitude lower. Hence, the development of energy buffering circuits that eliminate electrolytic capacitors while maintaining high energy storage density and high efficiency is one important requirement to achieving future grid interface systems that have both a small size and a high reliability.
It is known that film capacitors have a reliability and lifetime which is higher than electrolytic capacitors, but it is also known that film capacitors have considerably lower peak energy density than electrolytic capacitors (by an order of magnitude).
However, because film capacitors can be efficiently charged and discharged over a much wider voltage range compared with charge/discharge voltage ranges of electrolytic capacitors, for 120 Hz buffering, energy densities similar to those achieved with practical systems which utilize electrolytic capacitors can be achieved with high-reliability film capacitors, so long as a wide variation of the capacitor voltage can be used.
One approach to develop energy buffering circuits that eliminate electrolytic capacitors utilizes active filter blocks (essentially bidirectional DC-DC converters). The active filter block approach effectively utilizes film capacitors while maintaining a desired narrow-range bus voltage. While this approach is flexible in terms of it use, it unfortunately leads to low buffering efficiency if high power density is to be maintained, due to losses in the active filter.
Other systems have incorporated the required energy buffering as part of the operation of the grid interface power stage. This approach can offset a portion of the buffering loss associated with introduction of a complete additional power conversion stage, but still introduces high-frequency loss and is quite restrictive in terms of operation and application.
As is also known in the prior art, energy buffering can be employed in many non-line-frequency applications where there is a energy transferred between a first source or load having a slow rate of varying power and/or a limited instantaneous power rating (perhaps a dc source or load) and a second source or load that has a component of power that varies faster and/or to an instantaneous value larger than that desired to be sourced or absorbed by the first source or load. For example, such applications include interfacing a battery system (which is desired to be efficiently charged or discharged at a limited rate and with a limited peak power) to a mechanical system that requires rapidly varying power flow and perhaps large peak power (e.g., by using a power converter driving an electromechanical system such as a motor). In such a system, an energy buffer is desired to provide the local-time difference between the power sourced or absorbed by the first source or load and the second source or load (e.g., the difference between that desired for the battery and that required by the power converter and motor for the mechanical system). In such applications, an energy buffer may be provided by an ultracapacitor or energy buffer system including one or more ultracapacitors. Applications requiring energy buffering of the nature described here may include, without limitation, motor drives, electric and hybrid vehicle drive trains, cranes, renewable energy systems including wind and wave energy systems, active filter and reactive power compensation systems, traction systems, laser driver systems, electromagnetic launch systems, electromagnetic guns, electromagnetic brakes and propulsion systems, and power systems for implanted medical devices.